1. Open the application drawer and select Settings > Audio Profiles
2. Choose General (1) then select Edit (2) via the setting icon or by pressing the General option.
3. Select the required SIM card (1) then Media Storage (2) and confirm by selecting ALWAYS (3)
Please note that you mustn't select Custom Ringtones as a default choice because you won't be able to reverse this choice and restoring the smartphone would be the only mean.
4. Now you can select your ringtone and confirm with OK.
If you want to use your own ringtone, plug your device as a multimedia device (MTP) to your computer and paste your song in the Ringtones file folder.